This is a very common niggly problem especially in people who have desk jobs. It is very difficult to maintain good posture if you sit in the same position all day. If this is you try to move around more at regular intervals. Is it possible to get standing work stations or at least workstations designed to have your computer at the right height for you.
It is amazing much how better you will feel in as little as one week if you follow the advice in this link: